Circuit for monitoring cells of a multi-cell battery during charge

ABSTRACT

A circuit (M) to monitor and protect individual cells ( 12 - 1 . . . 12 -n) of a multi-cell battery ( 10 ) from over-charge and acquire data to be used to determine various characteristics of the cell state-of-health is connected to each individual cell ( 12 ) of a multi-cell battery as the battery is being charged. The circuit includes a portion (Q 1 -R 5 ) to bypass charging current from the cell, this portion being variably pre-settable to bypass current above a desired high voltage (V-By) limit for the cell. As the battery is being charged, the bypass circuit will shunt current around a cell when the preset high voltage level is exceeded, thus preventing any damage to the cell. The circuit (Q 2 ) can be operated to produce a pulse of current and the change in voltage of the monitored cell in response to the change in current, dV/dI, can be used to determine the cell internal resistance. The cell polarization resistance also can be determined. The data acquired can be used to determine factors relating to the cell state of charge (SOC) and its state of health. A programmable controller (C) controls all of the circuits and also acquires the data produced by the circuits.

GOVERNMENT STATEMENT

[0001] All or part of this invention was developed for Yardney and theU.S. Air Force under Government Contract No. F33615-98-C-2898. The U.S.Government may have certain rights to this invention under terms of thecontract.

FIELD OF THE INVENTION

[0002] The invention relates to a circuit for monitoring the voltage andcurrent of individual cells of a multi-cell battery during charging andto bypass charging current in excess of a pre-set value that can bevaried.

BACKGROUND OF THE INVENTION

[0003] Multi-cell, rechargeable batteries, such as those of the lithiumion type, are often used in mission control applications, such asuninterruptible power supplies and various military applications. Theoutput voltage of such batteries depends on the numbers of cellsconnected in series and the particular chemistry selected for the cells.In some applications, a sufficient number of cells can be connected toachieve voltages as high as 400V.

[0004] As a multi-cell battery is being charged or recharged, a currentsource is connected across all of the series connected cells. As thecharging takes place, individual cells might react differently to thecharging current. In particular, it is desired that a cell not beovercharged since this would damage the cell and perhaps even thebattery. Various circuits have been used to bypass excess current fromreaching an individual cell during a charging cycle so that it will notbe damaged. It is also desirable to monitor the state-of-health of eachof the battery individual cells and the composite battery. This involvesdetermining such parameters as the internal resistance, polarizationresistance, and remaining capacity of each cell of the battery as apercentage of original capacity measured in ampere-hours, often calledthe state-of-charge (SOC).

BRIEF DESCRIPTION OF THE INVENTION

[0005] The present invention relates to a circuit that can monitor andprotect individual cells of a multi-cell battery from over-charge andacquire data parameters to be used to determine various characteristicsof the cell state-of-health.

[0006] The circuit in accordance with the invention is connected to eachindividual cell of a multi-cell battery to be monitored as the batteryis being charged. The circuit includes a portion to bypass chargingcurrent from the cell, this portion being pre-settable in a variablemanner to bypass current above a desired high voltage limit for thecell. As the battery is being charged, the bypass circuit will shuntcurrent around a cell when the pre-set voltage level is exceeded, thuspreventing any damage to the cell.

[0007] In another aspect of the invention, the circuit can be operatedto produce a pulse of discharge current. When this is done the change involtage of the monitored cell in response to the change in current,dV/dI, can be used to determine the cell internal resistance. The cellpolarization resistance also can be determined by extending thedischarge pulse. The data acquired can be used to determine factorsrelating to the state of charge (SOC) of a cell and its state of health.

OBJECTS OF THE INVENTION

[0008] An object of the invention is to provide a circuit to monitor allindividual cells of a multi-cell battery during its operation.

[0009] An additional object is to provide a circuit to monitor anindividual cell of a multi-cell battery during charging and to bypasscharging current if a pre-set upper limit of the cell is exceeded duringbattery charging while permitting the charging of the other cells whichhave not reached the pre-set voltage limit.

[0010] Another object is to provide a monitoring circuit for individualcells of a multi-cell battery in which, as a cell is added to a batterypack, a monitoring circuit for the cell also is added in a modularfashion and interfaced to a controller in a modular fashion.

[0011] Yet another object is to provide a circuit to monitor individualcells of a multi-cell battery that can be operated to acquire data of acell that is indicative of its state-of-health and state-of-charge.

[0012] Still a further object is to provide a monitoring circuit foreach cell of a multi-cell battery that protects the cell againstovercharge by bypassing current after the cell is charged to a pre-setupper voltage limit and that can be operated to acquire data to be usedto determine the cell state-of-charge and state-of-health.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other objects and advantages of the present invention will becomemore apparent upon reference to the following specification and annexeddrawing in which:

[0014]FIG. 1 is a schematic diagram of a circuit in accordance with theinvention for monitoring a cell of a multi-cell battery as it is beingcharged and to bypass current in excess of a predetermined level.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Referring to FIG. 1, at the left side are shown the individualcells 12-1 . . . 12-n of a battery 10 of a battery pack. The cells 12are connected in series between a current charging source I, which canbe external to the battery pack and can be for example a solar cellarray, and a point of reference potential, such as ground 14. As many ofthe cells 12 are connected in series as needed to form the battery tohave a desired output voltage.

[0016] The invention is illustratively described with respect to alithium ion type battery. However, it is applicable to other types ofrechargeable batteries, such as lead-acid and nickel-cadmium. In suchbatteries, cells are added in series to obtain a battery having aspecified output voltage. For example: the average lithium cell voltageis 3.5V, so that eight cells connected in series make a 28V battery. A28V battery may have a high value limit of 36V and a discharge voltageof 20V. Batteries, such as of the lithium ion type, require carefulmonitoring and control of the voltage of each of the cells to some uppervoltage limit and some lower voltage limit. Thus, each individual cell12 of the battery is to be monitored during charging to measure itsvoltage. Also, as described below, the state-of-charge andstate-of-health of each cell also are to be determined.

[0017] In FIG. 1, the input leads 16 and 18 of a monitoring circuit Mare illustratively shown connected to the terminals of the positive andnegative electrodes of a cell, illustratively 12-3, of the battery.There is a separate monitoring circuit M for each cell and only one suchcircuit is described since each is the same. The cells 12 of the battery10 and the circuits M preferably are packaged in a battery pack. If thebattery pack is expanded with more cells, corresponding additionalcircuits M are provided. A cell monitoring circuit M is powered from thecells 12 themselves. The circuit M is designed to consume as littleenergy as possible to preserve the power of battery 10.

[0018] The operation of a circuit M is controlled by a controllerillustratively shown by block C. The controller C can be a programmableor pre-programmed microprocessor and has analog outputs to and inputsfrom the circuit M as described below. Controller C is integrated intothe battery pack with the monitoring circuits M. The controller Cinterfaces with each of the circuits M to set various operating points,monitor charge and discharge current, measure temperature, provideinformation to the external charger source, and control the battery packswitches to disconnect and protect against excessive charge ordischarge. The controller C has a serial interface to communicate with ahost computer, shown generally as H. The controller can have thenecessary ADC and DAC converters to interface with the monitor circuitM. The controller C uses the ADC and DAC converters mostly to interfaceto the monitor circuit M. An analog output from a DAC also can beprovided to control the battery charger current. Preferably, allcommunications outside the battery to the host computer H are done bythe serial interface. It is also possible for the battery charger to usethe serial interface instead of the DAC output.

[0019] In a typical application, the host computer relies on the batteryand controls all system components utilizing the battery. The hostcomputer is programmed with information such that it can do loadshedding or make other decisions as to how best utilize the remainingstored energy in the battery 10. For example, the battery pack withcontroller C can be in a satellite and the charging current source I besolar cells. The host computer would be the primary on-board computer incharge of all the satellite resources. The host computer H may providefurther data to the controller C to take advantage of various operatingmodes of the battery 10. In general, satellites and other space vehicleshave their own central computer. The controller C in the battery ispresumed to have the superior knowledge of the battery. It providesinformation to the host computer H of the satellite or other vehicle. Ina typical application, for example, if the host computer specifies theremaining life of the mission and wants the most power while sacrificingthe unneeded battery life, controller C will determine how to accomplishthat goal. In a satellite application, the satellite ground controlsystem typically would not control the battery operation. Such detailedoperation would only occur under extreme emergency measures. Of course,the application could be one in which the battery pack is charged from aconventional power source on the ground and the host computer andcontroller are hard wired to each other.

[0020] The leads 16 and 18 across the cell 12-3 monitored by circuit Mare connected to the upper and lower ends of a resistive divider formedby resistors R1 and R2. This divides the voltage of the monitored cell12-3 down for an input from the junction of the two resistors to thenon-inverting (+) input of an operational amplifier A2. Resistors R1 andR2 preferably are of high precision, such as 0.01%. At its power inputs,amplifier A2 obtains its negative rail voltage from the negativeterminal of the cell being monitored over line 18 and its positivevoltage from the positive terminal of the cell over line 16. Theoperational amplifier A2 preferably has high DC gain and low bandwidth.The input voltage range for amplifier A2 includes the negative supplyreference for the amplifier. For a lithium-ion battery, the cell voltagerange is from 2.5V to 4.5V, depending on the cell state of charge (SOC).The bandwidth of amplifier A2 is determined by the feedback signalprovided between its output terminal and the amplifier inverted input(−) terminal by a network of a parallel connected resistor R3 andcapacitor C1.

[0021] The operating voltages are chosen for amplifier A2 such that theamplifier will be disabled for voltages below a predetermined voltagefrom the monitored cell, this being about 3.5 volt in the lithium ionbattery example being described. This minimizes current drain byamplifier A2. As described below, because of the feature of being ableto disable amplifier A2, the bypass circuitry will not be turned ONaccidentally at low voltages of the monitored cell. The bypass circuitis only needed to shunt current above a specified voltage range of thecell being monitored, for example, between 3.5V and 4.5V for theillustrative lithium-ion cell.

[0022] The output of A2 is connected through a resistor R4 to the baseof a Darlington transistor Q1 which is configured as a common-emitteramplifier. The collector of Q1 is connected through a resistor R5 to theplus terminal of the monitored cell over line 16. Resistor R5 servesmultiple purposes. First, R5 acts as a load to dissipates the bypassedcurrent rather than requiring Q1 to dissipate all of the bypassedcurrent. Second, as described below, R5 serves as a precision currentshunt to measure the amount of the bypassed current. The value of R5 isselected so that the bypass current cannot exceed a safe upper limit. Ina typical application, the safe upper limit of the bypass current ischosen as 0.1C, (10% of the maximum charge current). The upper limit maychange for different kinds of cells under different conditions. Also,the largest expected upper limit can be set by the value of R5 and thelower limits be controlled by the controller C program.

[0023] An amplifier A1 has a signal input V-By at its non-invertinginput (+). Input V-By is a differential signal of variable predeterminedmagnitude from external control electronics in controller C that isreferenced to the logic ground of the controller C electronics. Theinverted (−) signal input of amplifier A1 is connected to the monitorcircuit logic ground at point 25. Voltage V-By is set by the controlelectronics to specify the high voltage limit at which each of the cells12 will be set. Amplifier A1 preferably is a high-common-mode, unitygain, precision difference amplifier and is powered from the cells 12.To provide the dynamic range required, amplifier A1 is powered at itspower inputs over lines 23 and 24 from two cell voltages higher and twocell voltages lower than the cell it is monitoring. For a monitorcircuit at the top cell of the stack, a voltage converter would be addedto produce a boosted voltage since there are no cells left from which toobtain the higher voltage. For the first two cells near ground 14, theA1 amplifier has its negative power supply terminal connected to theminus voltage supply (NISV), such as 15 volts, of an external voltagesource (not shown) since there are no two cell lower voltages at thebottom of the stack.

[0024] Amplifier A1 has a differential output whose negative referenceVOL is connected to line 18, the minus terminal of the cell beingmonitored. The other output VOH of A1 is connected to the inverted (−)signal input of amplifier A2. Amplifier A1 converts the bypass set pointvoltage V-By from the controller C ground reference to the reference ofthe cell being monitored. This common-mode difference voltage can be ashigh as 400V in a lithium-ion battery.

[0025] Amplifier A2 compares the voltage of monitored cell 12-3, asscaled by divider R1-R2 and taken from the junction of divider R1-R2,applied at the non-inverting (+) input of A2 with the pre-set V-By highlevel set point voltage from amplifier A1 applied as VOH at theinverting input (−). If the V-By limit is exceeded by the measured cellvoltage, then A2 produces a signal that turns on the Q1 bypasstransistor. When transistor Q1 is turned ON, the cell charging currentis bypassed around the monitored cell 12-3 over leads 16 and 18 throughQ1 and through the current shunt resistor R5. The bypass current isdesignated I Bypass. This effectively makes a precision hard voltagelimit on the voltage of the cell 12-3 being monitored.

[0026] It should be noted that V-By is adjustable and set from outsideof circuit M through controller C. This can be done by the hostcomputer. Thus, the set point is programmable and can be varied. Avariable set point, for example, as set by the main computer of asatellite, allows choosing higher voltages for cells. While this willshorten battery life, it may be a reasonable tradeoff when the usefullife of a mission will be achieved anyway.

[0027] As another example, charging of the battery 10 by solar cells fordifferent satellite orbits may make it desirable to use different fullcharge limits to compensate for the dark time of solar cells when theycannot view the sun. The provision of the variable set point permitssuch tradeoffs to be dynamically controlled by the remote host computer.

[0028] The current bypass resistor R5 is a precision resistor thatconverts the bypassed current I Bypass to a voltage. The voltage at themaximum positive voltage end of R5 is applied to the non-inverted (+)input of an amplifier A3 and the lower voltage end of R5 is applied tothe A3 inverted (−) input. Amplifier A3 preferably is a unity gain,high-common-mode, high-precision difference amplifier. Amplifier A3 runsfrom an external voltage source (not shown), for example, plus and minus15 volts, referenced to the controller logic circuitry ground. Thevoltage output IBP of amplifier A3 is a measurement of the bypasscurrent I Bypass. That is, amplifier A3 converts the I Bypass currentmeasurement from the cell reference voltage level and references it tothe controller C logic ground for use by the controller C and/or thehost computer.

[0029] As indicated, the controller C sets the value of the chargecurrent I. The battery 10 charge current I is at a value in terms of avoltage of a value that is known to the controller C. The controllerelectronics, for example its microcomputer, subtracts the current thatis bypassed by Q1, represented by the A3 IBP output voltage, from thebattery charge current I known to the controller to determine the netcurrent that charges the monitored cell 12-3. The controller C operatesto decrease the charge current I when I Bypass becomes greater than aspecified amount as determined by a control algorithm for the batterycell chemistry. The control algorithm is programmed into the controllerC.

[0030] As explained, the cell monitoring circuit M measures the cellvoltage and any bypassed current during charge. If a cell is beingovercharged, current is bypassed to keep the cell voltage from risingabove a preset high voltage limit. Preferably, there is a charge controlalgorithm in the controller that tells the charger when to reduce thecharge current so that it becomes unnecessary for the bypass circuit towaste a portion of the charge current. The specifics of the chargealgorithm are not the subject of this application, which is directed tothe interface circuitry to the battery pack.

[0031] An amplifier A4, which preferably is a high-common-mode, unitygain, precision gain difference amplifier, has its non-inverting (−) andinverting (+) inputs connected to the lines 16 and 18 of the cell 12-3being monitored. Amplifier A4 also operates from the external voltagesource (not shown), e.g. a plus (PISV) and minus (NISV) 15 volts,referenced to the controller C logic circuitry ground. Amplifier A4preferably is a very stable and high precision, (0.02%) beingachievable, amplifier. It converts the local cell 12-3 voltage to aground referenced signal for measurement by the controller. Thecontroller uses the output voltage of A4 to produce data used todetermine various characteristics of the state-of-charge andstate-of-health of the cell. Not shown is the temperature sensor for thecell. The state-of-charge is compensated for the temperature of thecell.

[0032] In general, the state-of-charge of a cell 12 is represented as apercentage of its full charge. Full charge occurs when the battery cellis at its upper voltage limit, 100% of full charge, i.e., 100% SOC. Fulldischarge, or 0% SOC occurs at the lower allowable voltage for a cell.If the cell voltage is half-way between these two voltage limits, it isat 50% SOC.

[0033] An opto-coupler Q2 is connected across R1 and an opto-coupler Q3connected across R2. The conduction state of each of Q2 and Q3 isdetermined by control logic signals S1 and S2 from the controller C,which signals control respective LEDs L1 and L2. While opto-couplers areshown in the preferred embodiment of the invention, it should beunderstood that any other conventional type of switching arrangement canbe used, for example, a transistor that is driven by a direct signal.

[0034] When opto-coupler Q2 is turned ON, preferably for a short time,by a pulse type signal S1 from the controller, the LED L1 is energizedto emit light. In response to the light from L1, the opto-coupler Q2conducts and shorts R1. This causes a positive going signal to beapplied to the non-inverting (+) input of A2 that produces a signal atthe output of A2 that turns on Q1 to its maximum allowed value for Ibypass. The pulse of current from Q1 appears across R5 and is applied toA3 to produce a voltage pulse output of IBP from A3 that is applied tothe controller C.

[0035] The pulse of current in R5 also causes a drop in the voltage ofthe monitored cell 12-3. This drop appears on lines 16 and 18, which arethe inputs to amplifier A4. The output of A4 is the voltage VCP. Thevoltages at the outputs of A3 and A4 are applied to the controller C andused to determine the internal resistance of the cell. The cell internalresistance (IR) is determined by dV (output of A4) divided by dI (outputof A3) in response to the Q1 current pulse S1. The controller C isprogrammed to compute IR or it sends the data to the host computer to dothis.

[0036] The cell polarization resistance (PR) can be determined byextending the length of the current pulse, i.e., the duration of thelight pulse from LED1. The current pulse is extended for a length oftime such that the cell IR rises to a higher value. The current pulse isterminated after the cell internal resistance stabilizes at the newhigher value of resistance. The final value of resistance minus theinitial value of resistance determines the polarization resistance. Herealso, the controller C is programmed to compute PR or it sends the datato the host computer to do this.

[0037] The controller C, through a corresponding monitor circuit M, canaccumulate data on each cell 12 in a battery pack. As the cells gothrough numerous charge and discharge cycles, the change in the internalresistance of each cell and change in the number of ampere-hoursdelivered by the cell by each cell, from the upper charge voltage limitto the lowest permitted discharge voltage, are measured andcharacterized. This data can be used to compute various characteristicsof a cell. For example, an increase in cell internal resistance anddecrease in ampere-hours are used to provide remaining capacity of thecell with respect to original capacity as a percentage to specify itsuseful residual life expectancy in a system.

[0038] The opto-coupler Q3 is turned ON by logic control signal S2 toproduce a pulse of light from L2 to cause Q3 to conduct and short outR2. This will hold Q1 in the off state and prevent current from beingbypassed. This feature is used to determine the state-of-health of acell.

[0039] The cell state-of-health is determined by different parametersthat characterize its degradation from its original manufactured cellcapacity. This determination can be made by determining a change in thecurrent ampere-hour capacity from its original known value. In any givencharge or discharge cycle, the controller C measures and computes anindication of the percentage of full charge that the cell is currentlyat. The algorithm for doing this is not part of the subject invention.It is sufficient to note that when the cell upper voltage limit isreached, the state-of-charge is 100%. When the cell is discharged to thelower permissible limit, the state-of-charge is 0%. By not equalizingthe cell voltage with the current bypass, the divergence of the finalstate of charge of the cell allows determining its state of health. Thisprovision can be used in the initial development of the algorithm. Forexample, an individual cell would be disabled from having the bypasscircuitry equalize it during charge. By equalizing all but a particularcell during charging, the observed changes in cell parameters, such asthe rate at which it accepts charge, can be used to characterize theefficiency of the equalization circuitry.

[0040] In addition to providing the percentage values, the controllercan be programmed to provide estimated times to reach charge ordischarge based upon the current passing through the battery cells.These factors give a real time view of the state-of-health andstate-of-charge of a cell.

[0041] Specific features of the invention are shown in the drawing forconvenience only, as each feature may be combined with other features inaccordance with the invention. Alternative embodiments will berecognized by those skilled in the art and are intended to be includedwithin the scope of the claims.

We claim:
 1. A circuit to monitor a cell of a multi-cell battery that isbeing charged from a current source, comprising: means for variablysetting a predetermined high limit voltage for the cell during itscharging; a first amplifier connected to the cell for receiving as oneinput the voltage of the cell as the battery is being charged and saidhigh limit voltage as another input, said first amplifier producing anoutput signal upon the cell voltage exceeding the high voltage limit;and a bypass circuit connected to the cell that is activated byreceiving the output signal produced by said first amplifier to bypassthe charging current around said cell.
 2. A circuit as in claim 1wherein said bypass circuit comprises a resistor through which thecurrent bypassed flows, and further comprising: a second amplifierconnected to said resistor to produce an output voltage corresponding tothe magnitude of the bypass current.
 3. A circuit as in claim 1 whereinsaid first amplifier further comprises a voltage divider connectedacross the cell and said one input to said first amplifier is taken froma point on said voltage divider.
 4. A circuit as in claim 1 wherein saidmeans for variably setting said predetermined high limit level voltagecomprises a programmable controller.
 5. A circuit as in claim 2 furthercomprising: switching means for operating said bypass circuit to producea pulse of discharge current; a third amplifier connected across thecell to measure the cell voltage in response to the current pulse; andmeans receiving the output of said second amplifier and said thirdamplifier for determining the internal resistance of the cell as afunction of the magnitude of the bypass current pulse and the cellvoltage response.
 6. A circuit as in claim 5 wherein said switchingmeans operates to extend the current pulse for a time after the cellinternal resistance is determined, said circuit further comprising meansfor using the outputs of said second amplifier and said third amplifierto determine the cell polarization resistance.
 7. A circuit as in claim5 further comprising: a controller for operating said switching means.8. A circuit as in claim 7 wherein said switching means comprises: alight source operated by said controller to produce light of a durationcorresponding to the duration of the current pulse, and an opto-couplerresponsive to the light to produce a signal to operate said firstamplifier to activate said bypass circuit.
 9. A circuit as in claim 8wherein said first amplifier further comprises a voltage dividerconnected across the cell and said one input to said first amplifier istaken from a point on said voltage divider, and wherein saidopto-coupler is connected across a part of said voltage divider tooperate said first amplifier to produce its output signal.
 10. A circuitas in claim 5 further comprising second switching means connected tosaid first amplifier to prevent said first amplifier from producing itsoutput signal for activating said current bypass circuit; and acontroller for operating said second switching means.
 11. A circuit asin claim 10 wherein said second switching means comprises: a lightsource operated by said controller to produce light; and an opto-couplerresponsive to the light to produce a signal to operate said firstamplifier to prevent said first amplifier means from producing itsoutput signal.
 12. A circuit as in claim 11 wherein said first amplifierfurther comprises a voltage divider connected across the cell and saidone input to said first amplifier is taken from a point on said voltagedivider, and wherein said opto-coupler is connected across a part ofsaid voltage divider to operate said first amplifier to preventproduction of its output signal.
 13. A circuit to monitor an individualcell or a multi-cell battery, said multi-cells being serially connectedto a current source, and said circuit comprising a first amplifierhaving power inputs connected to both terminals of the individual cellto be monitored, and a second amplifier having one power input connectedto a cell serially above said individual cell and having a second powerinput connected to a cell serially below said individual cell to providea dynamic operating range for said first amplifier, having a signalinput for determining the high voltage limit for said individual cell,and having an output connected to a signal input of said firstamplifier.
 14. A circuit in accordance with claim 13 further comprisinga bypass circuit connected across said individual cell, said bypasscircuit including a resistor and a bypass switch connected to the outputof said second amplifier.
 15. A circuit in accordance with claim 14further comprising a third amplifier connected across said resistor. 16.A circuit in accordance with claim 14 further comprising a voltagedivider connected across said individual cell to be monitored, saidfirst amplifier having a further signal input connected to a point onsaid voltage divider.
 17. A circuit in accordance with claim 16 furthercomprising first and second switch means connected to said voltagedivider for operating or preventing operation of said first amplifier.18. A circuit in accordance with claim 17 wherein said first and secondswitch means each comprise an opto-coupler connected across a portion ofsaid voltage divider.
 19. A circuit in accordance with claim 13 furthercomprising a network of a parallel connected resistor and capacitorconnected between the output of said first amplifier and said signalinput of said first amplifier to which the output of said secondamplifier is connected.